Monitoring Approach

Currently there is public concern about the idea of injecting CO2 and the possible dangers posed by leakage. Therefore for the moment the monitoring approach must consist of a technology with a proven track record for monitoring fluid movement and pressure changes in the subsurface, i.e. time-lapse seismic surveys. Once CO2 sequestration is demonstrated to be safe, cheaper monitoring alternatives may be more appropriate, such as gravity and satellite surveying.

The approach to developing a monitoring strategy described here is focussed on time-lapse seismic surveys, however this approach could be modified to examine other geophysical techniques. The sensitivity study described in this thesis not only indicates how well injection can be monitored, but should be used as a guide to what data should be collected and how it should be analysed. Here the sensitivity study is first put in context of where it should fall in the site selection process (Section 7.2.1), and then using the results presented in Chapters 4, 5 and 6, suggestions are made regarding what data should be collected depending on the monitoring requirements for a site. A cartoon can be seen in Figure 7-1 which demonstrates this workflow.

Figure 7-1 Cartoon illustrating the suggested site selection workflow. The ticks and crosses indicate the possible results from the screening and indicate the path which should be taken after the screening.

Site Selection Workflow

Screening Initial Assessment:

1. Storage Capacity Estimates 2. Geomechanical Studies 3. Seismic Sensitivity Study

 Use analogues, outcrops and theoretical models

Data Collection:

• Low Resolution seismic Survey

• Exploration/ Appraisal well

X

X

X

X

More detailed screening:

1. Storage Capacity Estimates 2. Geomechanical Studies 3. Seismic Sensitivity Study

 Identify potential issues with the reservoir

Use screening results to guide monitoring strategy

X

X

X

X

Monitoring Aim Near- offset Reflectivity Mid-offset Reflectivity Far-offset Reflectivity Converted S-Waves Attenuation Detecting CO2 migration Quantifying CO2 stored X X X

Differentiating gaseous and supercritical CO2

Distinguishing structural and residual trapping Detecting pressure build-up Determining the fluid distribution model.

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7.2.1 Site Selection

The ideas presented in this Section represent an idealised workflow for site selection based on the learnings from this research and do not necessarily represent the current, accepted approach. The selection of an aquifer for sequestration will be based on a number of well defined steps. Following the identification of a possible target formation there will be a site screening process, which will consist of storage capacity estimates to determine whether the site offers sufficient volume to make the project financially viable, geomechanical studies to determine safe injection pressures and a seismic sensitivity study to calculate if CO2 injection into the formation could be monitored using time-lapse seismic surveys.

Within this idealised workflow this screening process will be conducted twice. First there will be an initial assessment, and then if the site fulfils all the requirements there will be a secondary assessment. The first phase will use information from analogues, outcrops and theoretical models, and the examination of seismic sensitivity will be similar to the rock physics sensitivity study conducted in Section 5.4. If the reservoir is found to have sufficient storage capacity, be geomechanically stable and have rock properties which are conducive for successful seismic monitoring, then this will warrant further data collection. Data collection will be in the form of a low resolution seismic survey to map reservoir architecture and better quantify the reservoir volume, and a well to measure reservoir properties. This additional data can be used to conduct more detailed storage capacity estimates, geomechanical studies and seismic sensitivity studies. At this stage a seismic sensitivity study similar to that in Section 5.5 will be conducted, i.e. fluid-flow simulation results should be included. This assessment stage will identify any potential issues with or unique properties of the reservoir, such as possible compartmentalisation or mineralogy highly conducive of mineral trapping. This knowledge will then be used to guide the monitoring strategy deployed at the site.

7.2.2 Monitoring Strategy

Prior to injection commencing, a monitoring strategy should be designed which will guide the scope of any surveys, i.e. what data will be collected and how often this should occur. These parameters will influence the cost of any monitoring. This Section will demonstrate how the outcomes from the seismic sensitivity study can be used to guide this process of survey design, and potentially how it can be used to reduce costs by clearly indicating which data is required. Using the results from the seismic sensitivity study the monitoring aims can be clearly stated, i.e. if there is possible issues with compartmentalisation, then a monitoring approach is needed which can detect pressure build-up. Presented here is the best data which should be collected to fulfil specific monitoring requirements.

190  Detecting CO2 migration, if there is concern about flow into surrounding formations.

o Zero-offset reflectivity.

 CO2 invasion of the pore space will be indicated by a brightening of the reflectors and a pushdown of reflections below the plume.

 The saturation front can be mapped using a near-offset stack. o Mid to far offset reflectivity.

 Has a greater sensitivity to fluid content than the zero-offset response and would be collected if the sensitivity study indicated that the zero-offset reflectivity is inadequate.

 Quantifying the amount of CO2, to verify containment.

o No approach prove particularly successful, possible useful data:

 Shallow aquifer: converted shear-wave reflectivity or far-angle data.  Deep aquifer: attenuation which would require high frequency data and

spectral decomposition.

 Determining CO2 phase, if there is uncertainty about the temperature and pressure

conditions in the reservoir: o Mid-offset data.

 Cross-plotting the AVO attributes A and B.

 Quantifying the amount of CO2 retained via different trapping mechanisms. o Dissolution: no seismic property found to be sensitive.

o Differentiating structural and residual trapping:

 Shallow aquifer: potential for identified through mid-offset reflectivity (cross-plotting A and B).

 Zero-offset reflectivity, identify the interface between the plume regions.  Pressure build-up, if identified as an issue during simulation studies.

o Zero-offset reflectivity, identify pressure changes.

 Pressure build-up should brighten the reflectivity and result in increased pushdown of the base-reservoir reflection.

 Change in reflectivity away from the plume on the near-offset stacks indicative of pressure build-up.

o Mid-offset reflectivity, differentiate pressure and saturation changes  Cross-plotting of the change in the A and B AVO attributes.  Determining the appropriate fluid distribution model.

191 o Deep aquifers: use high frequency well-based techniques for the interwell region,

such as those used at Nagoaka.

o The model used may depend on the type of trapping in the reservoir:

 Structural trapping, (immediately following injection) should be modeled using the patchy response.

 Residual trapping, should be modeled as homogeneous.

 Due to the relative insensitivity of Vp to fluid distribution at high saturations, it may be appropriate for the homogenous model alone to be used.

This information is summarised in Table 7-1.

Table 7-1 Indicating the best seismic data to be collected to achieve certain monitoring aims. It is assumed that if non-zero offset reflectivity is collected so is the zero-offset reflectivity. The colours indicate the confidence assigned to the approach.. Green: Should Succeed, orange: May succeed, red: Unlikely this approach will work.

Monitoring Aim Zero- offset

Reflectivity Mid-offset Reflectivity Far-offset Reflectivity Converted S-Waves Attenuation Detecting CO2 migration

Quantifying CO2 stored X X X

Differentiating gaseous and supercritical CO2

Distinguishing structural and residual trapping

Detecting pressure build-up Determining the fluid distribution model.

X X

As the time-lapse method requires the subtraction of one survey from another, the acquisition parameters and processing sequence for the monitor surveys should be kept as similar as possible to that for the baseline survey (Lumley and Behrens 1998). This baseline survey should be conducted prior to injection. One method of assuring that acquisition is identical between surveys is the use of a permanent seismic array, such as that used successfully at the BP Valhall field in the Norwegian North Sea (van Gestel et al. 2008). These surveys should then be processed in an identical fashion, via parallel processing. van Gestel et al. (2008) presented a workflow for acquiring and processing time- lapse seismic surveys over a producing hydrocarbon field, this workflow could be modified and applied to monitoring sequestration. The study also discusses how seismic and well data can be used together to update both the static and dynamic reservoir models.

There are some practical limitations which may impact on this idealised monitoring approach. One is the issue of data quality and noise. For example at non-zero offsets the amount of noise will increase and the high frequencies will be attenuated, which may limit the angle range available for

192 interpretation. A second very important practical issue is that of cost. Time-lapse seismic surveys are commonly acquired over producing hydrocarbon fields, where the results influence decisions regarding wells and production strategy and therefore they can make a significant impact on profits. Whether this surveying method is financially viable at sequestration sites is another question. However the aims of monitoring at sequestration sites do differ from those at hydrocarbon fields, as during CO2 injection the aim is to detect leakage and potential problems, whereas at producing oil and gas fields the time-lapse data will guide production decisions. Therefore the frequency of surveys required at sequestration sites is likely to be lower than that at hydrocarbon fields, thereby reducing the costs.

The frequency of surveys will be dictated by a mixture of cost concerned and the fluid-flow simulation results showing plume migration and pressure build-up. From the modelling results in Chapter 5 it was predicted that after 2 years of injection the pressure would increase about 2 MPa and the plume would grow to about 1000m wide. These changes would both be detectable on seismic surveys. At Sleipner about 6 identical seismic surveys have been conducted in 12 years, roughly one every 2 years (Arts et al. 2008). The first repeat survey was after 4 years of injection and showed a large plume, and the subsequent surveys have shown this plume grow. The frequency of surveys at Sleipner appears adequate to monitor the plume growth.

The cost of monitoring will vary significantly with certain factors, such as whether it is terrestrial or marine and whether a permanent acquisition array is used. Hendriks et al. (2004) estimated that the cost of repeat seismic surveys (conducted every 5 years) was 0.3 US$ per tonne of CO2 stored. The overall cost of a sequestration site is highly variable (IPCC 2005), but estimates for onshore storage in Europe put the cost at about 2.8 US$ per tonne of CO2 stored (Myer et al. 2002). Therefore the cost of time-lapse seismic surveys is small in comparison to the cost of the overall project, and potentially much cheaper than the costs which could be incurred by unidentified leakage. Due to the nature of sequestration and the tight financial constraints the monitoring should be carefully planned in advance to minimise on unnecessary acquisition and processing. This cost reduction can be guided by the results from the seismic sensitivity study as demonstrated in Table 7-1.

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